In various uses of propagated electromagnetic radiant energy, efficacy demands that the radiated energy pattern be controlled. By way of example, an application of particular interest in this respect to makers of electronic article surveillance (EAS) systems is that the radiated energy pattern be confined to a specific area under surveillance, such as an exit area of a retail facility. Thus, to the extent that a radiated energy pattern extends beyond such exit area, i.e., overranges and thus fails to adequately fall off beyond a desired physical limit, the extended area cannot be used for such as article display purposes, since displayed articles in the extended area bearing EAS tags will be subject to alarm activity on receiving the radiated energy pattern.
EAS efforts heretofore known have looked extensively to measures to control overranging, e.g., the use of shielding to confine the radiated pattern to a confined area under surveillance, the use of a capacitive, on-floor pad, disposed between transmitting and receiving antennas, and plural transmitting antennas aside a controlled area, each transmitting respective complemental parts of an EAS tag activating message.
Further, various efforts have been forthcoming as to antenna array configurations which ar said to effect control of overranging.
A classic problem in EAS systems is thus that a transmitter antenna configuration that provides good EAS tag excitation may not pass stringent regulatory emission requirements. One solution, additional to those discussed above, is to lessen the field excitation level and bring the participating transmitting and receiving antennas more closely adjacent. This results in an essentially unmarketable system, i.e., not covering a desired extent of a controlled zone. Another avenue has been the use of a so-called "Figure-8" transmitting antenna, wherein the top and bottom coils are of opposed phase excitation. The limitation of the Figure-8 arrangement is, firstly, that the interrogation fields contain null zones which degrade detection sensitivity, and, secondly, that the manner in which the distant fields cancel each other depends on how closely the geometric centers of the participating coils are disposed.
Turning to specific antenna designs of U.S. patents, U.S. Pat. No. 4,751,516 is quite specific to the center feeding of a two-loop transmitting antenna,
U.S. Pat. No. 4,251,808 establishes as well-known an antenna having two outermost loops opposing a larger center loop, but requires the presence of a grounded shorted turn arrangement, wherein the cross-over shield portions are insulated from the shorted turn, e.g., as is seen at 60 and 62 in FIG. 5 thereof.
U.S. Pat. No. 4,260,990 calls for a transmitting antenna adapted for coupling to a transmitter and having at least one loop lying in a plane, a receiving antenna adapted for coupling to a receiver and having at least two twisted loops lying in a common plane, each loop being twisted 180 degrees and in phase opposition with each adjacent loop. The antennas have a different number of loops and a mutual magnetic coupling therebetween and the receiving antenna has an effective total loop area of one phase equal to the effective total loop area of opposite phase.
U.S. Pat. No. 4,243,980 relates to three twisted loops in each of the transmitting and receiving antenna systems.
U.S. Pat. No. 4,769,631 discloses a transmitter antenna configuration that is coaxial and coplanar, with inner and outer loops in additive phasing, using elliptical coils rotated with respect to each other to create a sheared field along the horizontal plane.
The antenna configuration of U.S. Pat. Nos. 4,510,489 and 4,510,490 is used hereinafter as a comparative base for the antenna system of the subject invention.
Other patents dealing with antenna structures of interest include French Patent No. 763,681, U.S. Pat. Nos. 2,597,518, 3,182,314, 4,135,183 and 4,859,991.
From applicants' viewpoint, none of the foregoing patents effectively addresses the problem of reducing distant field levels and, at the same time, not affecting the interrogation zone field levels in an appreciable manner.